Biofuel technologies for the commercial fleet
While gasoline drives consumer mobility, it’s diesel that we turn to for the heavy-duty vehicle (HDV) fleet. Diesel fuel use accounts for roughly 20 percent of U.S. petroleum use and has an even greater share in European markets. Although ethanol is the primary biofuel used to displace gasoline, biofuel development for HDVs is centered on renewable diesel and biodiesel, two distinctly different technologies with unique production pathways.
First, we need to talk about diesel engines. Diesel engines are fundamentally different from gasoline engines – they use a different thermodynamic cycle. While your gasoline-powered passenger vehicle relies on the spark plug to initiate fuel combustion, diesel engines make use of a high-compression stroke to heat up the fuel enough to drive combustion. This difference in engine cycles makes diesel engines heavier, noisier, and typically more efficient than their otto cycle counterparts. Because diesel engines are a lot heavier and more resilient, they tend to be more expensive, but they also tend to be more forgiving of what is combusted inside of them.
Due to this heavy construction and high compression, many liquid hydrocarbons can be combusted in diesel engines, which opens up interesting avenues to explore for biofuels. Fuel options range anywhere from burning used fryer grease, to more advanced reformation processes that produce a fuel indistinguishable from conventional diesel. Each of these technologies has different advantages and disadvantages. However, when it comes to commercial scale, biodiesel and renewable diesel are top candidates.
The production of biodiesel is carried out by chemically converting a low-value grease or oil, such as used fryer grease, to a higher-value hydrocarbon chain. While cooking oil may be combusted in diesel engines directly, the performance characteristics and blending possibilities increase by converting the oil to something more similar to diesel. This typically takes place through a transesterification process, although other processes such as pyrolysis exist.
Transesterificaiton is a relatively simplistic chemical conversion process, in which an alcohol, like methanol, reacts with the cooking oil (a branched ester) to create a mono-alkyl ester (biodiesel) and glycerol (a byproduct). The biodiesel that is formed is similar to diesel that exists today; however, it has slightly lower energy density and slightly different combustion properties from conventional diesel. Biodiesel also has different solvent properties that can lead to rubber gasket and line deterioration. These characteristics contribute to concern over the usability of biodiesel due to potential engine deterioration, impacting manufacturer warranties and vehicle performance. Many diesel engine manufacturers, however, have started increasing warranty coverage from B5 blends (5 percent biodiesel, 95 percent conventional diesel) to B20 and even B100 blends (100 percent biodiesel).
Beyond blending considerations, biodiesel adoption also suffers from sustainability and food crop concerns. Biodiesel use in Europe has triggered controversy over palm oil production, in which forests in developing countries are replaced by palm monocultures due to incentives for biodiesel procurement. Similarly, in the U.S. soybean oil is extensively used to fulfill the biodiesel production requirement, which is derived from an animal feed crop. While a significant fraction of biodiesel may be derived from spent cooking oil, aggregation and purification of these oils may be time-intensive and difficult, and the total supply is limited. Further, as biodiesel requirements increase, the value added from biodiesel production may far exceed cooking oil production, contributing to an overall increase in the price of food.
While renewable diesel does not necessarily circumvent sustainability and food-pricing concerns, it does avoid the blend wall constraint. Renewable diesel at the consumption level aims to simulate conventional diesel, and so may be treated as such. The difference between renewable diesel and biodiesel is the production pathway. While biodiesel takes oils and upgrades them through a transesterification process, renewable diesel utilizes a hydro-treating process, which makes use of a hydrodeoxygenation reaction at elevated temperature and pressure in the presence of a catalyst to effectively convert pant oils to straight-chain molecules similar in composition to conventional diesel.
This process is more difficult to carry out, and is accordingly more energy intensive and often more carbon intensive than the biodiesel production counterpart. In turn, this may translate to greater costs relative to biodiesel production.
While the light-duty vehicle fleet has cellulosic ethanol to turn to for the next generation of low-carbon, non-food-crop biofuel, biodiesel has the analogous algae-diesel pathway. Algae can be genetically modified to produce a significant fraction of oil – which can then be collected from the algae and converted into biofuels. This is an extensive area of research.
In theory, algae are easy to cultivate, can grow with limited attention almost anywhere (as anyone with a poorly ventilated bathroom can attest), and can make use of water that is unfit for human consumption. While the use of algae for low-cost biodiesel production is technically feasible, many barriers exist for economic production of algal biodiesel. These barriers include land area and land-use considerations, algal cultivation unit design, and oil separation and extraction processes.
Algae growth rates are highest when the concentration of algae in solution is low, yet oil yields may be higher for high algae concentrations. In turn, separation of oil from these algae-water slurries is energy intensive and expensive, and an ideal algae cultivation unit design remains uncertain. Overall, extensive research and development will be required to bring costs down and make scalable, consistent processes for algal biofuel production. As algae become more efficient at producing these desirable oils, and the extraction and cultivation methods decrease in cost, the opportunity to utilize algae for large-scale diesel fuel production becomes realistic.
As with all biofuels, technological viability is ultimately determined by cost and scale. Today, petroleum-derived diesel fuel provides a level of service and value to the heavy-duty vehicle fleet that is unrivaled. While biodiesel has experienced some adoption in European and U.S. markets, the long-term use and success of biofuels will depend on research and development, and the technological advances that come from that.
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